The list is likely to have been compiled from multiple data breaches, according to Hold Security

A cybersecurity company said Tuesday it has obtained a list of 360 million account credentials for Web services, likely collected through multiple data breaches.

Analysts with Hold Security came across the credentials during their work over the last three weeks while studying underground forums where stolen data is for sale, said Alex Holden, chief information officer with the Wisconsin-based company.

"This month has been very fruitful for hackers," he said in a phone interview.

One batch of 105 million details, discovered about 10 days ago by the company, included email addresses and corresponding passwords, but it isn't clear what Web services the credentials unlock, he said.

It is possible the data came in part from data breaches at dating or job-related sites, which would have large numbers of users, although it has not been confirmed, he said.

"We don't know who has been breached," Holden said. "Ultimately, we are trying to figure out who the players are."

Those who are unemployed and using job sites could be more vulnerable to spam and phishing schemes since they are more likely to respond to offers or to emails from people they don't know.

Hackers frequently collect logins and passwords by compromising computers with malicious software. But Holden said that method of attack is unlikely due to the "sheer amount of data" and considering how many computers would have to be exploited.

The volume of data collected is "a sign that hackers are switching their tactics," focusing on large stores of data such as those held by companies rather than targeting individual users.

Data security has become a hot-button issue after the breaches at the retailer Target and Neiman Marcus, both of which have said malicious software was installed on their point-of-sale terminals and collected shoppers' unencrypted payment card details.

Other companies, including hotel management company White Lodging Services and the arts and crafts store chain Michaels, are investigating possible breaches after fraud was detected on payment cards used by their customers.

Hold Security has also found 1.25 billion email addresses circulating among hackers, with a batch of 156 million addresses collected on Friday, Holden said. The list, he said, would be useful for spammers. Data thieves often try to sell such email lists on underground forums.

Holden's company sells a paid service that alerts companies when analysts find their stolen data.